The proposed project comprises mechanistic research to assess the biogeochemical interactions that affect bioavailability of chlorinated benzenes (CBs) during in situ remediation of CB-contaminated groundwater and sediments using a dual-biofilm barrier approach. The dual-biofilm barrier innovatively combines both an anaerobic dehalogenating consortia and aerobic oxidizing bacteria, seeded on granular activated carbon (GAC), to achieve complete transformation of higher chlorinated benzenes to carbon dioxide, which otherwise commonly stalls at formation of monochlorobenzene and benzene upon reductive dechlorination of higher chlorinated benzenes. The overall goals of this proposed project are to further the scientific and technical advancement of this innovative technology and to demonstrate its effectiveness in protecting ecological and human health by treating contaminants in situ and reducing their mass flux to surface water or in subsurface plumes that are potential drinking water sources. Laboratory and field tests will be conducted using a Superfund site where dense non-aqueous phase liquid (DNAPL) CB contamination is present in wetland sediments and groundwater, and preliminary studies have been performed on the contaminant distribution and degradation processes. The effectiveness of this biobarrier technology to serve as a long-term remedy at this Superfund site and other hazardous waste sites depends on several factors, including biogeochemical interactions and dynamics with the biofilm, other biobarrier components, the underlying sediment, and the inflowing groundwater. These factors will be investigated through studies designed to address five specific aims: (i) determine the stability and effectiveness of aerobic and anaerobic biofilms on GAC through microcosm experiments under shock loading conditions and employing electron microscopy and surface spectroscopy to characterize the surfaces; (ii) examine the interactions between unseeded GAC and the site matrix (sediment and water) in batch experiments to assess contaminant sorption / desorption kinetics by utilizing aqueous phase speciation and surface characterization methods; (iii) investigate biodegradation processes and rates of CBs in replicate upflow column experiments that compare unseeded GAC to GAC seeded with only an anaerobic culture and both the aerobic and anaerobic cultures to assess the dual-biofilm effectiveness in treating CBs; (iv) assess the impacts of different electron acceptors and other biogeochemical conditions on degradation rates of CBs in upflow column experiments to establish conditions that optimize barrier efficiency and performance; (v) evaluate on-site performance of the dual-biofilm barrier through field tests to determine biofilm effectiveness and sustainability over a multi-year period under realistic hydrologic and biogeochemical dynamics and environmental conditions.